The present invention relates to vehicle tyres and in particular, but not exclusively, to a passenger car tyre.
The vast majority of road vehicles, and particularly passenger vehicles, use a form of pneumatic tyre as the interface between the vehicle and the road surface. The characteristics of the tyre vary dependent on the road surface, the type of vehicle and the type of use of the vehicle. For example, a tyre for a heavy goods vehicle (HGV) will have a deep side wall relative to the tyre width in order to accommodate the weight of the vehicle and load. In contrast, a tyre for a performance car will have a shallow side wall in relation to the tyre width in order to minimise the wheel displacement relative to the road surface. A family saloon might have a tyre of intermediate side wall depth in order to provide improved comfort for the passengers of the vehicle, whilst still providing acceptable levels of vehicle handling performance.
However, a common feature of all tyres is a trade off between the grip generated between the tyre and the road surface (a function of the tyre coefficient of friction), and the rolling resistance of the tyre. Whilst increased grip is clearly beneficial with respect to vehicle safety, the increase in grip inevitably leads to an increased rolling resistance of the tyre, which can make the vehicle more noisy and less efficient when driving in a straight line.
Developments have been made to tyre compounds in order to optimize the trade off between tyre friction and rolling resistance. However this design point remains an issue for tyre designers attempting to improve the fuel consumption of the associated vehicle whilst maintaining tyre grip for cornering ability and vehicle safety.
One proposed solution to this problem is to provide a vehicle tyre assembly having a tyre formed from non-uniform tyre compound. Accordingly, the central portion of the tyre is made of a harder tyre compound which has a lower friction coefficient and correspondingly lower rolling resistance. Arranged on either side of the central portion are areas formed of a compound with higher coefficients of friction and correspondingly higher rolling resistance. As the vehicle speed increases (for example, consistent with the vehicle travelling in a straight line) the centrifugal growth associated with high speed rotation causes the central portion of the tyre to expand outwardly allowing the main contact point of the tyre with the road to be provided by the central hard compound section. In this way the rolling resistance of the tyre is reduced thereby reducing the fuel consumption of the car and at the same time reducing the wear of the softer compound, high friction sections on either side of the central portion.
However, it has been found that the relative centrifugal growth is often insufficient and the softer compound sections remain in contact with the road and wear accordingly. This in turn reduces the benefits of the variable compound tyre.
In order to overcome this problem, prior art devices teach the use of an actuator in order to additionally extend the central portion in a radial direction. However this solution is complex and presents problems in terms of operating the device upon sudden braking at high speed in that the actuator is unlikely to be able to retreat from the inner portion of the tyre sufficiently quickly to allow the outer soft compound portions to contact the road to provide for rapid braking.
Additionally, known systems require an external pressure source in order to operate. This adds substantial complexity to the system. Furthermore the use of an actuator to extend the central portion adversely affects the ride quality of the tyre since tyre compliancy is greatly reduced by the application of a high pressure over a low surface area of the tyre.
It is an object of the current invention to at least mitigate some of the above problems.